You are not their target market, you have a top tier phone and Wifi/internet at home.
They want to catch those users who still have Gingerbread or Honeycomb and get them to their bottom tier phone.
Oh and BBM has a better read and deliver report, if you have someones pin it is easy to start chatting, whatsapp does have some problems with contacts not appearing in the client.
If you asked a girl for her number or her pin at a bar the pin is the better option because a number you cant lose but with the pin if the girl still has some control - if she doesnt like you she can just delete you and you wont have her as a contact. Speaking from experience
I know there is some scanning benefit to it as well but never did it so cant say for sure - like a like on FB you can get say 5fm's pin then they can broadcast specials and bands shows to you. . . .